Processes for preparing nitrile rubbers by emulsion polymerization of (meth)acrylonitrile with conjugated dienes, for example butadiene, and optionally, small amounts of other comonomers are widely known, for example, from DE 658 172. One type of such other co-monomers are α,β-unsaturated carboxylic acids and the resulting terpolymers are often abbreviated as “XNBR”. In addition, it is known from U.S. Pat. No. 3,700,637 that nitrile rubbers of this type can be hydrogenated, preferably using homogeneous rhodium halide complex catalysts. The strength of the products thus obtained is substantially improved by comparison with the strength of non-hydrogenated starting products.
It is further known that catalysts based on a metal of Group VIII of the periodic table supported on a porous carrier such as carbon, silica or alumina can be used to catalyse hydrogenation of the carbon-carbon double bonds of conjugated diene polymers. In U.S. Pat. No. 4,337,329 it is e.g. disclosed that the hydrogenation of acrylonitrile/butadiene copolymers can be achieved by using palladium in combination with another metal precipitated together on a porous powdery or granular carrier such as silica, silica-alumina, alumina, diatomaceous earth, or activated carbon. It is described that the polymer may be hydrogenated as such, however, that good results are in particular obtained if the polymer is used in the form of a solution.
In U.S. Pat. No. 4,452,951 the hydrogenation of acrylonitrile/butadiene copolymers using a hydrogenation catalyst supported on silicon dioxide having a specific surface area of not more than 600 m2/g and an average pore diameter of from 80 to 1,000 Å is described.
In U.S. Pat. No. 4,853,441 the hydrogenation of acrylonitrile/butadiene copolymers was carried out using Pd supported on a non porous alkaline earth metal carbonate carrier, preferably on CaCO3.
In Applied Catalysis A: General 177 (1999) 219-225 it is disclosed to use palladium supported on mesopore size-controlled smectites for hydrogenation of butadiene-acrylonitrile copolymers in carbon tetrachloride.
In U.S. Pat. No. 5,612,422 the hydrogenation of high molecular weight polystyrene in the presence of silica supported Rh or Pt containing catalysts was reported.
The carrier materials proposed so far for the heterogeneous hydrogenation catalysts used for hydrogenating diene based polymers often suffer from having low mechanical strength which in consequence results in the disadvantage, that they cannot be successfully used several times, since, owing to the mechanical stress a fine grain portion occurs, which either renders the hydrogenation product impure or results in a working-up step substantially more difficult.
It is furtheron known from Macromolecules 36 (2003), 4294-4301 (Orietta Monticelli et al.) to use palladium nanoparticles supported on so called hyper-branched aromatic polyamides (aramids) for the hydrogenation of small molecules, i.e. unsaturated organic chemicals (benzene, benzylideneacetone, phenylacetylene, diphenylacetylene and quinoline). It was found that the NH2 groups of the HB aromatic polyamides bond Pd(II) ions. Hyper-branched polymers (“HB polymers” for short) are highly branched macromolecules with a three-dimensional architecture which pursuant to Prog. Polym. Sci. 29 (2004) 183-275 belong to the class of dendritic polymers. Over the past 15 years, HB polymers have attracted increasing attention owing to their interesting properties and greater availability as compared with other types of dendrimers. In Prog. Polym. Sci. 29 (2004) 183-275 it is described that HB polymers have potential applications in coatings, as additives, for drug and gene delivery, as macromolecular building blocks, for nanotechnology and supramolecular science. However, only few published reports and patents exist relating to catalysts supported on HB polymers: In Adv. Synth. Catal., 345 (2003), 333 (Mecking et al.) it is described to use hybrids of palladium nanoparticles with highly branched amphiphilic polyglycerol as a catalyst for the hydrogenation of cyclohexene. The reaction was performed in a continuously operated membrane reactor and the reaction mixture was continuously drawn from the reactor via the membrane. In Macromol. Chem. Phys. 208 (2007), 1688 (Mecking et al.) it is further described that hydroformylation of 1-hexene may be performed with rhodium colloids stabilized by poly(ethylene imine)amides with a hyper-branched polyamine core and a lipophilic periphery.
The object of the present invention was to provide a new and improved hydrogenation process using a heterogeneous catalyst allowing the selective hydrogenation of a diene-based polymer with a high degree of hydrogenation within short reaction times at low reaction pressures and mild temperatures. It was a further object to find a catalyst disposing of sufficient mechanical strength which may be easily recovered and often re-used in such hydrogenation of diene-based polymers.